296 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



into two tubes. This is continued in the anterior part of the truncus by a horizontal 

 septum (short in urodeles, longer in anura) separating aortic and pulmonary trunks, 

 the former subdivided in a similar way a little farther forward into carotid and aortic 

 portions. 



In the early larvae of the amphibia each fully developed aortic arch except the 

 last extends into the gills, but as the branchiae begin to be absorbed, a small vessel 

 connecting the afferent and efferent arteries at the base of each gill enlarges and 



FIG. 300. Heart and adjacent parts of Protopterus, after Rose a, atrium; aoe, 

 oesophageal artery; /, air bladder (lung); c, conus; h, hepatic vein; ji, is, superior and 

 inferior jugular veins; oe, oesophagus; pa, pulmonary artery; pc, postcardinal vein; ph, 

 pharyngeal artery; s, sinus venosus; sc, subclavian vein; 1-4, afferent branchial (aortic) 

 arteries. 



becomes the path of the main blood stream and a part of the arch of the adult (fig. 

 304). Of these four arches 3, 4, 5, and 6 of the primitive scheme the fifth is 

 lost in the adults of all except a few urodeles and caecilians. The fourth connects 

 with the dorsal aorta and the sixth with the pulmonary arteries. These last, which 

 often have a ductus Botallii, are noticeable for the large cutaneus arteries anterior 

 and posterior which arise from them and which play an important part in respira- 



